1. Field of the Invention
The present invention relates to a semiconductor pressure sensor for detecting pressure and a manufacturing method thereof.
2. Description of Related Art
In a conventional semiconductor pressure sensor, a diaphragm portion having a thin thickness is formed in a silicon substrate. A plurality of gauge diffusion resistive layers (strain gauges) are formed on a surface of the diaphragm portion. As a result, displacement of the diaphragm portion is detected by strain gauges.
In this semiconductor pressure sensor, it is suggested that an electro-chemical etching is carried out with respect to the silicon substrate to accurately control the thickness of the diaphragm portion. In detail, a wafer is prepared by forming an n-type epitaxial layer on a p-type silicon substrate. When a diaphragm portion is formed, an anisotropic etching is carried out with respect to the p-type silicon substrate using an aqueous solution such as potassium hydroxide (KOH) or the like, in a state where a reverse voltage is applied to a region where the diaphragm portion is to be formed. At this time, because a pn junction in the wafer is reverse-biased, a depletion layer extending from the pn junction to a silicon substrate side is created. An front end of the depletion layer is exposed to the etchant, as the etching process advances. When the depletion layer is exposed to the etchant, the etching of the silicon substrate is ceased due to a difference in potential between the silicon substrate and the etchant. In this way, since the position at which the etching of the silicon substrate is ceased is specified by the thickness of the depletion layer, the diaphragm portion can be accurately formed.
In the semiconductor pressure sensor, a region (diaphragm formation region) where the diaphragm portion is formed in the epitaxial layer, is formed as an island region which is electrically insulated from the epitaxial layer (peripheral region) encompassing the diaphragm formation region. A plurality of (for example, four) gauge diffusion resistive layers are formed on a surface of the diaphragm formation region, and an integrated circuit is formed in the peripheral region. The plurality of gauge diffusion resistive layers are connected to form a bridge circuit. The integrated circuit supplies voltage to the bridge circuit. Therefore, the bridge circuit generates a voltage signal in correspondence with the displacement of the diaphragm portion. The potential of the diaphragm formation region is fixed by an aluminum wire running from the integrated circuit to the diaphragm formation region.
When the electro-chemical etching as described above is carried out, if current leaks from the integrated circuit formed in the peripheral region into the diaphragm formation region, the etching of the silicon substrate cannot be ceased at a desirable position. Therefore, for example, JP-A-6-45618 teaches that a diode is disposed in the aluminum wire for fixing the potential of the diaphragm formation region, which runs from the integrated circuit to the diaphragm formation region. The diode can prevent leak current from flowing from the integrated circuit into the diaphragm formation region. FIG. 5 is a schematic plan view of the semiconductor pressure sensor as described above.
FIG. 5 shows one semiconductor pressure sensor chip 101 among a large number of semiconductor pressure sensors formed in a wafer. In a diaphragm formation region 102, a plurality of gauge diffusion resistive layers (not shown) is formed and connected to make up a bridge circuit. In a peripheral region around the diaphragm formation region 102, an integrated circuit portion 103 is formed. Power supply voltage is supplied to the integrated circuit 103 via a pad 104.
A conductive pattern 105 for feeding voltage used for carrying out the electro-chemical etching is formed along scribing lines in a peripheral portion of the sensor chip 101. The conductive pattern 105 is electrically connected to the diaphragm formation region 102 by way of an aluminum wire 106. The aluminum wire 106 and the pad 104 are connected to each other by way of an aluminum wire 108. Diodes 107, 109 are disposed in the aluminum wires 106, 108, respectively.
Because the sensor chip 101 is structured as described above, when the electro-chemical etching is carried out, a positive voltage for creating a depletion layer is applied from the conductive pattern 105 to the diaphragm formation region 102 via the aluminum wire 106. At this time, the diode 109 prevents current from flowing into the integrated circuit portion 103 via the aluminum wire 108 and the pad 104. That is, leak current flowing into the integrated circuit portion 103 can be prevented by the diode 109. It is to be noted that, when the wafer is cut up (diced) into plural sensor chips along the scribing lines, the conductive pattern 105 is separated from each sensor chip.
When the semiconductor pressure sensor is brought in an operating state, voltage is supplied to the diaphragm formation region 102 via the pad 104, diode 109 and the aluminum wire 108. The potential of the diaphragm formation region 102 is fixed by the voltage thus supplied. Because the diode 107 is provided in the aluminum wire 106, it is possible to prevent leak current from flowing from the diaphragm formation region 102 to the conductive pattern remaining at the peripheral portion of the sensor chip 101.
The integrated circuit portion 103 has a power supplying circuit for supplying electric power to the bridge circuit formed by the gauge diffusion resistive layers. As one example of the power supplying circuits, JP-B-62-55629 teaches a constant current circuit which supplies constant current to the bridge circuit by disposing a resistor in a power supply line connected to the bridge circuit and controlling current flowing through the resistor to a constant value. In this case, a maximum potential applied to the gauge diffusion resistive layers is lowered from the voltage of the power supply line (power supply voltage) by a voltage drop at the resistor. Therefore, when the potential of the diaphragm formation region 102 is fixed by the power supply voltage supplied via the pad 104 and the diode 109, the fixed potential becomes higher than the maximum potential applied to the gauge diffusion resistive layers. As a result, it is possible to prevent current from leaking out of the gauge diffusion resistive layers.
The inventors of the present invention considered a power supplying circuit having a circuit structure shown in FIG. 4, as the power supplying circuit provided in the integrated circuit portion 103. In the power supplying circuit shown in FIG. 4, a transistor 301 is provided on a ground side of a bridge circuit 200 formed by gauge diffusion resistive layers 201-204, and a power supply side thereof is directly connected to a power supply line L. Resistors 302, 303, an operational amplifier 304, a variable resistor 305, and a transistor 306 constitute a constant current circuit. The constant current circuit causes constant current to flow between the collector and emitter of the transistor 306. As a result, current proportional to that constant current flows between the collector and emitter of the transistor 301, i.e., through the bridge circuit 200. In this case, the value of the constant current flowing through the bridge circuit 200 is adjustable by adjusting a resistance value of the variable resistor 305 by means of trimming or the like.
According to the power supplying circuit as shown in FIG. 4, a value of voltage applied to the bridge circuit 200 can be increased comparing to the power supplying circuit in which the resistor is disposed in the power supply line connected to the bridge circuit. Therefore, even when, for example, a dry battery is used as a power supply source, output voltages V1, V2 of the bridge circuit 200 can be made large.
However, in the power supplying circuit shown in FIG. 4, the maximum potential of the gauge diffusion resistive layers 201-204 becomes equal to a power supply voltage Vcc. The potential of the diaphragm formation region 102 is lowered from the power supply voltage Vcc by a forward voltage drop of the diode 109. For this reason, the maximum potential of the gauge diffusion resistive layers 201-204 becomes higher than the potential of the diaphragm formation region 102, thereby causing leak current flowing from the gauge diffusion resistive layers 201-204. If the leakage of current from the gauge diffusion resistive layers occurs, detection sensitivity of the pressure sensor lowers.